The quality of a large number of industrial products (medicines, explosives, colorants, pigments, cosmetics, polymers, catalysts, chemical products for agriculture, etc.) depends on the physical characteristics (measurement, size distribution, homogeneity, morphology, etc.) of the particles making them up. In conventional manufacturing processes finely divided particles are obtained by means of a sequence of production stages: crystallization or precipitation, drying, triturating or grinding, and homogenisation. Implementing any of these stages can be very costly for compounds that are unstable at high temperature or which undergo degradation when subjected to mechanical action. The development of methods for obtaining finely divided particles in a single production step is thus of considerable technological interest.
There exist in the state of the art various methods that relate to obtaining finely divided solid particles.
On one hand, there are methods which use solutions under pressure. Notable amongst the most widely used of such methods is that described in the U.S. Pat. No. 4,582,731, known as the RESS Procedure, and the method described in international patent WO 9003782, known as the GAS Procedure.
On the other hand, there are methods in which cooling of the solution to be crystallized is caused by the evaporation of a volatile fluid. Notable among the most widely used are the methods described in U.S. Pat. Nos. 4,452,621 and 5,639,441, known as the DCC Procedure.
There follows a brief description of the mechanism used in each one of the procedures mentioned to obtain finely divided particles.
Firstly, in the RESS (Rapid Expansion of a Supercritical Solution) Procedure the solid to be precipitated is first dissolved in a fluid at a pressure and temperature higher than the critical temperature and pressure. This supercritical solution is then expanded rapidly at atmospheric pressure, with the resulting precipitation of micrometric particles.
In this method, supersaturation of the compound in the solution to be precipitated is due to the rapid fall in the solvating power of the supercritical gas caused by the sudden reduction of pressure.
Secondly, in the GAS (GAS Anti-Solvent) procedure, the gas or fluid under pressure acts as an anti-solvent on a liquid solution of a compound C to be precipitated. Initially, said compound to be precipitated is dissolved in a fluid A in order to form a solution A, and the solution A is then mixed at a pressure P, with a second fluid B, for example CO2, which is converted into a gas when the pressure is reduced to atmospheric pressure. In this method the fluid A and the fluid B have to be totally miscible at the pressure P. In the GAS procedure, however, the fluid B acts as an anti-solvent, and precipitation of the compound C takes place during mixture of the solution A with the fluid B, at pressure P and temperature T.
Thirdly, there exists a method in which the cooling of the solution to be crystallized is caused by the evaporation of a volatile fluid, which is known as the DCC Procedure.
In a DCC (Direct Contact Cooling) procedure, the evaporation of a volatile fluid (refrigerant) is used to provide the coldness necessary to permit precipitation. In this case, the degree of homogeneity of supersaturation throughout the entire solution to be crystallized depends of the quality of mixing between the solution and the refrigerant liquid. The better the mixture the more homogenous the cooling will be, and the narrower the distribution of particle size obtained.
In this crystallization process the solvent in which the compound to be precipitated is dissolved and the refrigerant fluid are not miscible and, therefore, a solution is not formed before precipitation. In this case, moreover, the solution A and the fluid B come into contact once the fluid B has been depressurised.
One major difference that should be highlighted between the methods of the prior art and that of this application is that precipitation of the compound of interest takes place using a mixture and not using a solution that contains the compound to be precipitated, as is the case in this invention.